This invention relates to fuel pressurizing and metering means for a small high speed turbine type of power plant and particularly to the electromechanical interface of an electronic fuel control suitable for missile or automotive applications.
As is well known in the art centrifugal gas turbine engine pumps have not heretofore been feasible because they require high cranking speeds to provide sufficient fuel system pressurization for starting the engine. It is also well known that where a centrifugal pump is directly driven by the engine, such a pressurization system employs a positive displacement pump for low engine cranking speed and requires a mechanical reduction drive. Additional attempts of hybrid pumping systems have employed a positive displacement pump for cranking in conjunction with a mechanical or hydraulic decoupler. The mechanical drive reduction necessitated by the positive displacement pump or the mechanical hydraulic coupling devices are not only expensive but they are also complex and heavy which add to the overall weight of the aircraft. Obviously, in aircraft applications any additional weight introduces a penalty to the system and hence, adversely affects the payload.
We have found that we can take advantage of the pressurization capabilities of the centrifugal pump by combining it with an electrically driven positive displacement pump for obtaining both starting and metering capabilities. The electric driven positive displacement pump speed becomes proportional to the desired fuel flow and thereby becoming the metering element. Motor driven pumps have not been conducive to system miniaturization in the past because the motor required two to three horsepower to deliver two gallons per minute at 800 or 900 pounds per square inch absolute. According to this invention the centrifugal pump is utilized to provide pressure thereby reducing the work required by the metering pump drive motor to a small fraction of the horsepower since the pressure load is reduced to essentially fuel line losses.
According to this invention the centrifugal pump is utilized in combination with either a piezoelectric driven metering pump or an electrical motor driven pump which may be of the vane type.
In both instances the metering becomes an integral part of the electronic control systems which meters fuel as a function of the output of the electronic control which serves to regulate fuel as a function of the demands of the engine requirement.